Basic gravity for entities

This is the gravity similar to minecrafts one.
I'm not sure whether there are other things to consider like falling through blocks, but the velocity changes are done.

Even if this PR won't get pulled it may help other contributors to implement that feature.

Is there a template like Bukkit uses one for PRs?

Just for reference are my sources of that magic values:
https://github.com/Bukkit/mc-dev/blob/master/net/minecraft/server/EntityLiving.java#L1025-L1134
https://github.com/Bukkit/mc-dev/blob/master/net/minecraft/server/EntityArrow.java#L290-L336
The rest of the TODO should be removed